Purity and choosing separation methods

A pure substance has a sharp melting point: it melts at a particular temperature rather than over a broad range. A mixture melts over a range of temperatures. Impurities also commonly lower the melting temperature compared with the pure substance.

To interpret melting point data, look at both the temperature at which melting starts and the temperature at which it finishes. Compare these with the known melting point of the pure substance, if it is provided.

For example, consider these results for two samples of a substance whose pure melting point is 80 °C:

Sample B is not simply ‘a substance with a lower melting point’: it melts across several temperatures. That range is the key evidence of impurity.

Choosing a method means finding a physical difference

Separation methods work because the components of a mixture have different physical properties. First identify what is present and what you want to collect.

A solution forms when a solute dissolves in a solvent. In salt solution, salt is the solute and water is the solvent. A dissolved solid needs a different separation method from an insoluble solid such as sand.

Filtration: collecting an insoluble solid

Filtration separates an insoluble solid from a liquid or solution. For example, it can separate sand from water, or collect an insoluble solid formed during a reaction.

Place filter paper in a funnel above a collecting beaker, then pour in the mixture. The liquid passes through the paper, but the undissolved solid is trapped. The solid left on the paper is the residue; the liquid collected below is the filtrate.

Dissolved substances pass through with the liquid. Filtering salt solution therefore does not separate the salt from the water.

Crystallisation: collecting a dissolved solid

Crystallisation is used to obtain a dissolved solid from its solution. Copper sulfate solution is a useful example because copper sulfate is more soluble in hot water than in cold water.

Gently heat the solution in an evaporating basin so that some water evaporates. This makes the solution more concentrated. Continue until it is saturated: it contains as much dissolved solute as it can hold at that temperature. Crystals forming on a clean, cold glass rod dipped into the solution can indicate that it is ready.

Leave the concentrated solution to cool. As the temperature falls, less copper sulfate can remain dissolved, so crystals form. Filter off the crystals, wash them with a little cold distilled water and allow them to dry. Cold water removes remaining solution while limiting how much of the crystals dissolves.

Simple distillation: collecting the solvent

If you want the water from salt solution rather than the salt, use simple distillation. It separates a solvent from a solution by vaporising the solvent and then condensing its vapour.

Heat the salt solution in a distillation flask. Water vaporises, while the dissolved salt remains in the flask because it does not vaporise at the temperature used. The water vapour enters a condenser, where a surrounding flow of cold water cools it. It condenses into liquid water and runs into a collecting vessel. The collected liquid is called the distillate.

As water leaves the flask, the remaining salt solution becomes more concentrated. Evaporation alone would lose the water to the surroundings; distillation allows it to be collected.

Fractional distillation: separating mixed liquids

Fractional distillation separates liquids that mix together and have different boiling points, such as ethanol and water. Ethanol boils at about 78 °C, whereas water boils at about 100 °C at atmospheric pressure.

The apparatus includes a fractionating column between the heated flask and the condenser. As vapour rises through the column, repeated condensation and vaporisation improve the separation. The vapour reaching the condenser first is richer in the lower-boiling liquid, ethanol. It cools and condenses, allowing an ethanol-rich fraction to be collected.

As the lower-boiling component is removed, the temperature rises. Different fractions can then be collected separately. The important distinction is that simple distillation is suitable for collecting a solvent from a dissolved solid, whereas fractional distillation improves the separation of mixed liquids.

Paper chromatography: separating soluble substances

Paper chromatography is suitable for separating a mixture of soluble substances, such as the dyes in an ink. The components travel at different rates as a solvent moves through the paper.

Put a small sample spot on a pencil line near the bottom of chromatography paper. Stand the paper in a suitable solvent with the sample line above the solvent level. As the solvent rises through the paper, it carries the soluble components with it. Their different movement rates separate them into spots.

This method is useful for separating and investigating small samples of dyes. It is not the method to choose when you want to collect a beaker of water from salt solution.

Applying the choice to an unfamiliar mixture

Use the properties of the components, not just the mixture’s name:

Sometimes more than one method is needed. To separate sand, salt and water, first filter out the insoluble sand. The filtrate still contains dissolved salt. You can then crystallise the salt or distil the water, depending on which substance you want to collect.

• Pure substance: sharp melting point.
• Mixture: melts over a temperature range, commonly lower than the pure substance’s melting point.
• Compare the start and finish temperatures and any known pure melting point.

Select the separation method

Choice depends on the desired product: crystallise to collect a dissolved solid; distil to collect its solvent.